Effects of the misalignment and axial gap on performance of a cover-plate pre-swirl system with impellers for gas turbine cooling

Abstract

Impellers are designed and used in the cover-plate cavity of the pre-swirl system of modern gas turbines to keep the circumferential velocity of the airflow consistent with the local velocity of the rotor. This reduces the aerodynamic loss of the pre-swirl air delivery system for turbine cooling and further improves pre-swirl system performance. However, in practical applications, the cover-plate disk with impellers is usually installed and fixed to the turbine disk by buckles, which leads to the possibility of the circumferential position of impellers deviating from the design intention during installation and a certain axial gap between impellers and the turbine disk, resulting in deviations in cooling performance. In this work, low-radius cover-plate pre-swirl systems with and without impellers are compared through numerical simulations, and the effects of the impeller circumferential displacement angle (θ) in the range of −0.5° to 0.5° and the axial gap (Δ) in the range of 0%–25% of the cover-plate cavity width are investigated on the flow characteristics and system cooling performance. Results indicate that the impellers are sensitive to the manufacturing and installation deviations. The non-designed circumferential displacement of the impeller will misalign it with the rotating receiver holes and supply holes, hindering the airflow movement in the radial direction and increasing the back pressure of the impeller. Consequently, the severe vortices on the suction side downstream of the impeller leading edge can occur and the entropy production can increase. Numerical results show that when the impeller is displaced by 0.5° circumferentially, the temperature drop effectiveness decreases significantly, and is about 21% lower than that at the designed condition. Moreover, while the axial gap is small, the overall flow pattern and system temperature drop are only slightly affected. As the axial gap increases, there are more cooling airs flowing through the axial gap and not subject to the work of the impeller. Therefore, the temperature drop starts to fall and the swirl ratio declines. For the system studied, the temperature drop effectiveness decreases by around 7.8% when the axial gap reaches 25% of the cover-plate cavity width.